This command sets up a general highlight, with the items of
canvas canvas to highlight in this manner
identified by tagOrId and the cmd prefix providing the implementation, i.e. the how to
perform the highlight.

This form is invoked when the mouse has entered (one of) the
item(s) the highlight was set up for. The callback now has to
perform any reconfiguration necessary to highlight the item
(group).

The result of the command can be anything. It is stored by the
system as client information and passed unchanged to the
un-highlight callback for its use. In this manner the highlight
callback is able to maintain custom state from highlighting to
un-highlighting.

Note that the callback does not have to maintain state, nor does
it have to actually reconfigure the item (group). In the latter
case the callback simply serves as easy enter/leave
notification.

This document, and the package it describes, will undoubtedly
contain bugs and other problems. Please report such in the category
canvas of the Tklib Trackers. Please
also report any ideas for enhancements you may have for either
package and/or documentation.